1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly this invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and develop strong cohesive bonds to various substrates, particularly aluminum, steel and glass, that the compositions are in contact with during curing.
2. Background Information
Organosiloxane compositions that cure by a hydrosilation reaction between alkenyl hydrocarbon radicals and silicon bonded hydrogen atoms are well known. The curing reaction is typically catalyzed by a metal from the platinum group of the periodic table or a compound of one of these metals, and the consistency of the cured materials range from gels to elastomers to resins. The ability of these compositions to cure at temperatures from ambient to about 200.degree. C. without the evolution of by-products make them desirable for a wide range of end use applications, including fabricated articles and encapsulating materials. A disadvantage that has delayed wider acceptance of this type of organosiloxane composition as coating materials is the relatively poor adhesion to organic and metal substrates developed during curing of the compositions.
Attempts have been made to improve the adhesion of organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction by coating the substrate with a primer prior to applying the curable organosiloxane composition or by adding an adhesion promoter to the curable composition. Organosilicon compounds containing a variety of reactive groups have been used as both primers and adhesion promoters for curable organosiloxane compositions. Silanes that have been used for these purposes typically correspond to the formula ASi(OR).sub.3, where R represents a lower alkyl radical. A represents a group capable of reacting with the substrate during curing of the organosiloxane composition, and includes but is not limited to ethylenically unsaturated groups such as vinyl radicals and 3-(meth)acryloxypropyl groups, epoxy-containing groups such as gamma-glycidoxypropyl, aminoalkyl, mercaptoalkyl, isocyanato and alkoxy groups. Organosiloxanes containing at least one of these reactive groups per molecule have also been used as adhesion promoters.
Adhesion promoting organosiloxanes containing two or more types of reactive groups per molecule are described in U.S. Pat. No. 4,912,188, which issued on Mar. 22, 1990; in Japanese Laid Open Patent Application No. 3/234,768; and in U.S. Pat. No. 4,732,932, which issued on Mar. 22, 1988.
Primer compositions and adhesion promoters comprising mixtures or reaction products of two or more organosilicon compounds containing at least one of the reactive groups discussed in the preceding paragraphs are also known. Mixtures of previously known adhesion promoters include the combination of an epoxy-substituted silane and an organosiloxane containing at least one vinyl radical and at least one silicon-bonded hydroxyl group described in U.S. Pat. No. 4,087,585; the combination of a 1,2-bis(trialkoxysilyl)ethane and an organosilicon compound containing at least one hydroxyl group or suitable hydrolyzable precursor and a group that reacts with the 1,2-bis(trialkoxysilyl)ethane or the curable organosiloxane composition, described in U.S. Pat. No. 4,737,562; and liquid vinyl-containing organosiloxanes prepared by the hydrolysis of a mixture of vinyltriethoxysilane and vinyltrichlorosilane in Japanese Laid Open Patent Application No. 72/36,255, published on Sep. 12, 1972.
A variety of catalysts have been used to promote a hydrolysis-condensation reaction involving the alkoxy and/or hydroxyl groups present on the adhesion promoter molecules and reactive groups present on the surface of the substrate. Suitable catalysts include but are not limited to tetraalkyl titanates and chelates such as acetylacetonates derived from metals including but not limited to aluminum, copper and titanium.
A disadvantage of using as adhesion promoters alkoxysilanes or organosiloxanes containing either vinyl or allyl as the silicon-bonded reactive group is the inhibiting effect of these compounds on the platinum-catalyzed hydrosilation reaction used to cure the organosiloxane composition. This inhibition is particularly evident at curing temperatures below about 120.degree. C.
One objective of this invention is to improve the adhesion to a variety of substrates exhibited by organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction.
A second objective is to define a class of organosilicon compounds containing at least one silicon-bonded higher alkenyl radical and at least one alkoxy group as the reactive groups. The compounds function as adhesion promoters for organosiloxane compositions curable by a platinum-catalyzed hydrosilation reaction without substantially inhibiting curing of the composition at temperatures below about 150.degree. C.